Lighting using various light source is activities or functions to brighten a specific place with some purpose and is widely used to brighten the environment at night or in dark places.
FIG. 1 is a cross section view of a flat plate illuminating device according to an example of a prior art. In FIG. 1, the illuminating device of the prior art includes a light source 10, louver or reflector 20. The light source 10 uses an incandescent lamp, LED, CCFL, etc. In FIG. 1, angles of the light shown in dotted lines are passed to the person, thereby causing visual discomfort. Such a illuminating device may reduce the UGR, but doses not become aesthetic or perfect flat plate lighting.
FIG. 2 is a cross section view of the flat plate illuminating device according to an example of the prior art.
In FIG. 2, a illuminating device 30 includes a diffusion plate 40 diffusing the optical source 10 and the light emitted from the optical source 10. The light emitted from the optical source 10 is emitted into the outside through the diffusion plate 40. The diffusion plate 40 reduces a hot spot of the optical source 10 and is used to evenly emit the light. Although the diffusion plate 40 is used, as shown in FIG. 2, the light of angles shown in dotted lines still gives discomfort to the human eye. That is, the diffusion plate 40 scatters the light up to the direction occurring highly the UGR, thereby causing a glare, such that the user's eye become tired, which does not meet criteria of indoor flat plate illuminating device.
Thus, it is important to reduce the glare in the indoor flat plate illuminating. The degree of displeasure due to the glare is represented by a constant called the UGR (Unified Glare Rating). That is, the UGR is value quantifying the degree giving the displeasure to the humans using the lighting.
When assuming the direction facing from the ceiling disposed with the illuminating device to the bottom side as 0 deg and the direction parallel to the ceiling as 90 deg, the UGR is calculated as the value of speed of light becoming 65 deg˜90 deg That is, when reducing speed of light of 65 deg˜90 deg, the glare is reduced. In Europe and Unites States, the illuminating device having the UGR of 19 or less is used as the indoor illuminating device.
As such, the current most indoor flat plate illuminating device may use the reflector or the louver, and reduces the angles of the light to be spread to a wide range that affects the UGR by embedding the whole illuminating device. In the prior art, although the diffusion plate is used, it is possible to reduce the effect of the hot spot but which is still not acceptable for the UGR standard of 19 or less.